Motorcycle Repair: ZX9 too lean?, kawasaki manual, winter mn

Question
Hi Mark,
My buddy has a '96 ZX9 with 21K miles on it. I'm trying to give him a hand with a running problem he's having. Last summer he took it into the dealer for a tune up and when he got it back, it never ran worse (choke req'd to keep it running, occasional backfire, etc). He parked it for the winter (MN), but of course, the problem is still with him. I've been through a number of bikes inside and out so I told him I'd take a look at it. Here's the status of what I found.

It takes full choke to make the thing run. Naturally, I checked out the fuel supply. Good smelling gas, petcock flows well, fuel filter is clean, fuel pump works enthusiastically, fuel lines are all clear to the float bowls. I pulled the carbs and dropped the float bowls, they all look clean, no dirt, no obstructions, no gummy gas. The jetting is stock according to the Kawasaki manual. The slide diaphrams are all in good condition and the slides appear to work smoothly on visual inspection. Choke assembly works smoothly. I found the two inner vacuum plugs loose on the intake manifold, but no change occurred when I tightened them. The vacuum lines for the outboard cylinders are in good condition and I tried pinching them off to see if that would effect a change. No change.  

The missing and rough running occurs at any RPM when I try to reduce the choke to anything less than FULL. It's obviously starved for gas or has an abundance of air, but I can't figure where it's coming from. Unless I missed something very minor in the carbs, I'm out of ideas. Can you offer any suggestions? Thanks.
Brian
Minneapolis, MN

Answer
Hi Brian,

I appreciate the detailed information. That seems to be reare with questions. Thanks! : )

I suspect the carburetors need cleaned. With time, gas varnishes and obstructs jets and fuel circuits. When the pilot jets/circuits become obstructed, MC engines only idle on choke. This is because the pilot circuit and enrichment (choke) circuit share passageways. When the pilot jet is obstructed, the choke when on compesates and becomes the piolt jet.

Remember, every pinhole inside the carbs has an important function and is critical for proper operation. Clean thoroughly.

I need your email address as requested. I'll return several files to assist you with th repair. You'll appreciate these files as a MC tech. Send to my email: mshively1@woh.rr.com

Respectfully,
Mark Shively